The present invention relates to an electrostatic inkjet recording method and apparatus for ejecting an ink composition using an electrostatic field. More specifically, the present invention relates to an inkjet recording method and apparatus for a high resolution capable of stably ejecting fine ink droplets.
According to electrostatic inkjet recording, a predetermined voltage is applied to each ejection portion of an inkjet head in accordance with image data, using an ink composition (hereinafter, simply referred to as ink) prepared by dispersing charged colorant particles in a dispersion medium, whereby the ejection of ink is controlled using an electrostatic force, and an image corresponding to image data is recorded on a recording medium. As a recording apparatus adopting the electrostatic inkjet recording, for example, JP 10-230608 A discloses an inkjet recording apparatus that ejects ink from a surface of a flat insulating substrate in a direction normal to the surface.
FIG. 6 shows a conceptual diagram of the inkjet recording apparatus disclosed by JP 10-230608 A during ejection of an ink droplet R. An inkjet head 80 includes a head substrate 82, ink guides 84, an insulating substrate 86, control electrodes 88, a counter electrode 90, a DC bias voltage source 92, and a pulse voltage source 94.
In the insulating substrate 86, nozzles (through-holes) 96 for ejecting ink are formed. The head substrate 82 is provided in an arrangement direction of the nozzles 96, and the ink guides 84 are disposed on the head substrate 82 at positions corresponding to the through-holes. The ink guides 84 extend through the nozzles 96, and each tip end portion 84a protrudes upward from the surface of the insulating substrate 86 on a recording medium P side. Each ink guide 84 is provided with a slit-shaped ink guide groove 83 in a vertical direction in the figure, and an ink composition is guided to the tip end of the ink guide groove 83 by a capillary phenomenon.
The insulating substrate 86 is placed at a predetermined distance from the head substrate 82, and a flow path of ink Q is formed therebetween.
The ink Q containing fine particles (colorant particles) charged in the same polarity as that of a voltage to be applied to the control electrodes 88 circulates in an ink flow path 98, for example, from the right side to the left side in the figure owing to a circulation mechanism of ink (not shown), whereby ink is supplied to each nozzle 96.
The control electrodes 88 are provided in a ring shape so as to surround the circumference of each nozzle 96 on the surface of the insulating substrate 86 on the recording medium P side. Furthermore, the control electrodes 88 are connected to the pulse power source 94 generating a pulse voltage in accordance with image data, and the pulse power source 94 is grounded via the DC bias power source 92.
Furthermore, the counter electrode 90 is placed at a predetermined distance from the tip ends of the ink guides 84 and grounded. The recording medium P is held on the counter electrode 90 so as to be opposed to the tip end portions 84a of the ink guides 84.
In such electrostatic inkjet recording, under the condition that only a bias voltage is applied to the control electrodes 88 by the DC bias power source 92, the Coulomb attraction with respect to the charged particles (charged particles, colorant particles) in ink, ink viscosity, surface tension, repulsion force between charged particles, fluid pressure in ink supply, and the like act on each other owing to the bias voltage, whereby ink rises slightly at the ink guide tip end to form meniscus.
Furthermore, the charged particles migrate to move to the vicinity of the meniscus by virtue of the Coulomb attraction and the like. That is, ink is concentrated.
When the pulse voltage (control voltage) by the pulse power source 94 is applied to the control electrodes 88, and an ejection ON state is established, a pulse voltage is superimposed on a bias voltage. Consequently, the ink Q present at the tip end of the ink guide is attracted to the recording medium P (counter electrode) side to allow meniscus to glow into a substantially conical shape, a so-called Taylor cone.
When a predetermined period of time elapses after the start of the application of a voltage, the Coulomb attraction acting on the charged particles and the surface tension of a dispersion medium become out of balance. Consequently, ink is ejected and flies toward the recording medium P as droplets, and attracted to the recording medium P side.
In the electrostatic inkjet recording, generally, a pulse voltage is modulated and applied to each control electrode 88 to control ejection ON/OFF, whereby ink droplets are modulated to be ejected, and on-demand ejection of ink droplets in accordance with an image to be recorded is performed. Thus, a desired image is formed on the recording medium P.
The inventor of the present invention has studied the ejection principle of such electrostatic inkjet recording, and found the following. As described above, when a pulse voltage (control voltage) is applied to the control electrode 88, meniscus grows at the tip end of the ink guide. Furthermore, when a finite time elapses, a large electrostatic force acts on the meniscus tip end portion with the highest electric field intensity, and the Coulomb force mainly acting on particles and the surface tension acting on the solvent become out of balance. At this time, a narrow liquid column with a diameter of about several μm to tens of μm called a thread extending toward a recording medium is formed. Then, when a finite time further elapses, the thread is separated in the middle into droplets, which are ejected toward the recording medium. Since the diameter of the thread is very small, the droplets formed by the separation of the thread are very minute. Thus, fine dots of about 10 μm can be formed on the recording medium.
Furthermore, the inventor has also found that the separation of the thread occurs at a frequency higher than a driving frequency of a pulse voltage for ejecting ink. That is, the separation of the thread continuously occurs a plurality of times within the time when a pulse voltage is applied once. Thus, one dot on the recording medium is formed of fine droplets ejected after the separation of the thread.
When one attempts to realize a higher resolution, it is desired to stably perform the formation and separation of the thread to stably form fine droplets. However, the formation and separation of the thread are based on a very complicated mechanism. Therefore, there is a possibility that until the thread is formed, the diameter and length of the thread, the separation position of the thread, and the like may vary. Those variations are considered to vary a dot diameter or a dot shape, and to cause the degradation of an image.